The present invention relates to optical monitoring devices and in particular to a micro-resonator which is coupled to an optical device to produce an electronic signal when the wavelength of the light provided in the optical device is within the resonance bandwidth of the micro-resonator.
Optical monitors are used with many optical devices. They may be used, for example, to determine the wavelength and/or optical power of light produced by a semiconductor laser in order to tune the laser. Optical monitors may also be used to determine the losses in an optical system, such as a electro-absorptive modulator or semiconductor optical amplifier, by measuring both the input energy and output energy of the system either over a broad band of wavelengths or in a specific narrow band. In the materials that follow, it is assumed that the light to be measured is propagating through a waveguide or is generated in an optical gain medium. In the material that follows, the term “waveguide” includes both traditional waveguides and gain media. Furthermore, the term light is used to indicate any radiation that may be transmitted via an optical waveguide.
An important use for optical monitoring systems is in tuning communications lasers. Communications lasers operating in a dense wavelength division multiplexing (DWDM) system are desirably especially finely tuned to be able to provide the closely spaced channels defined for this standard. Exemplary channels for a DWDM system are defined as νn=ν0±ndν, where ν0 is the central optical frequency, (e.g. 193.1 THz) and dν is the channel spacing (e.g. 100 GHz or of 50 GHz).
Typical semiconductor lasers are able to be tuned in a range of 30–40 nm while maintaining acceptable power levels. Tunable semiconductor lasers may be a distributed feedback (DFB) laser, a distributed Bragg reflector (DBR) laser or other laser that uses distributed mirrors. Tunable lasers may also be more conventional lasers having a resonant cavity that includes at least one Fabry-Perot cavity as a reflector. In each of these lasers, the resonant wavelength may be tuned by electrically or thermally adjusting the “optical length” between the reflectors. The optical length may be adjusted by changing the actual length and/or the index of refraction of the material between the reflectors. Although not explicitly described herein, pressure, as applied by one or more piezoelectric elements, may also be used to adjust the index of refraction.
A typical laser tuning system couples the light provided by the laser is coupled to a waveguide. A portion of the light traveling through the waveguide is tapped, for example, by splicing an optical fiber through the cladding of the waveguide. This tapped light may be applied to one or more optical filters that separate light having a particular wavelength and then to an optical sensor, such as a photodiode. The light tapped from the waveguide may also be directly detected by an optical sensor to determine the power level of the laser. Splicing optical fibers to the output fiber to tap the light may cause undesirable power loss or scattering of light that may result in increased noise or feedback into the laser.
The signals provided by the optical sensors may be applied to control circuitry that adjusts the temperature of the laser or its reflectors or adjusts an electrical potential applied to the reflectors. This circuitry changes the resonant wavelength of the laser to center it within a predetermined communications channel.
Thus, considerable circuitry, separate from the semiconductor laser, is typically used to tune the laser.